1. Field of the Invention
The present invention relates to a dynamic pressure bearing device which allows a lubricating fluid to generate a dynamic pressure and supports a shaft member and a bearing member by such dynamic pressure in such a manner that the two members can be relatively rotated with respect to each other, and a method for manufacturing such dynamic pressure bearing device.
2. Related art
In recent years, there have been proposed various kinds of dynamic pressure bearing devices which are used to rotationally support various rotary bodies such as a polygon mirror, a magnetic disk, an optical disk and the like. In the dynamic pressure bearing device of this type, a dynamic pressure bearing surface situated on the shaft member side and a dynamic pressure bearing surface on the bearing member side are disposed in such a manner that they are opposed to each other in the radial direction of the dynamic pressure bearing device with a given clearance between them; and, a lubricating fluid such as air, oil or the like poured or charged into the clearance, when the shaft member or bearing member is rotated, is pressurized due to the pumping action of the dynamic pressure bearing surfaces of the shaft member and bearing member to thereby generate a dynamic pressure and thus the two members, that is, the shaft member and bearing member can be supported by the thus generated dynamic pressure of the lubricating fluid in such a manner that they can be relatively rotated with respect to each other.
In some of the dynamic pressure bearing devices of this type, as means for generating such dynamic pressure, there is used a dynamic pressure generating groove which is formed in a herringbone shape, a spiral shape or the like. However, especially, in the case of a journal bearing device, there have been conventionally proposed a step dynamic pressure bearing device and a taper dynamic pressure bearing device in both of which the above-mentioned dynamic pressure generating groove is not used. Here, FIG. 7 shows a structure of an ordinary step dynamic pressure bearing device. In this structure, as shown in FIG. 7, in the inner peripheral wall surface of a bearing member 2 which surrounds the periphery of a shaft member 1, in more particular, at a plurality of portions (in FIG. 7, at three portions) thereof, there are respectively disposed a plurality of dynamic pressure generating portions 2a each including a projecting surface which projects in a step-shaped manner toward the central side of the bearing member 1.
While the dynamic pressure generating portions 2a are disposed intermittently along the peripheral direction of the bearing member 2 in two or more (in FIG. 7, three), in narrow spaces respectively formed between these dynamic pressure generating portions 2a and the outer peripheral surface of the shaft member 1, if a lubricating fluid such as oil, air or the like can be squeezed and thus pressurized, then there can be generated bearing dynamic pressures respectively in their corresponding narrow spaces. By the way, in the taper dynamic pressure bearing device, the above-mentioned dynamic pressure generating portions 2a each including a projecting surface do not project in a step-shaped manner but project through a continuously inclined surface (tapered surface).
In FIG. 7, in a flow direction (in FIG. 7, in the counterclockwise direction) of the lubricating fluid shown by an arrow mark, on the downstream side of the respective dynamic pressure generating portions 2a, there are disposed negative pressure cancel portions 2b respectively formed of recess-shaped separate grooves which are formed so as to be continuous with their associated dynamic pressure generating portions 2a. These negative pressure cancel portions 2b respectively consisting of recess-shaped separate grooves are formed in such a manner that they are suddenly depressed from their associated dynamic pressure generating portions 2a toward the outside of the radial direction of the bearing member 2; and, since the lubricating fluid is allowed to flow into enlarged spaces defined by these negative pressure cancel portions 2b, there can be avoided the formation of useless negative pressure areas. That is, if the negative pressure cancel portions 2b respectively consisting of recess-shaped separate grooves are not formed, there can be generated a negative pressure, thereby causing external disturbance. This external disturbance lowers the load capacity of the step dynamic pressure bearing device to thereby reduce greatly the dynamic pressures in the radial direction of the step dynamic pressure bearing device, which makes it impossible for the step dynamic pressure bearing device to obtain a good dynamic pressure characteristic.
As described above, the negative pressure cancel portions 2a are components which are indispensable to both of the step dynamic pressure bearing device and taper dynamic pressure bearing device. And, in order to be able to obtain a good dynamic pressure characteristic, preferably, the negative pressure cancel portions 2a may be as deep and narrow as possible. Normally, they are formed so as to have a depth of 20 .mu.m or more and, when forming such deep and narrow groove, generally, there is employed a cutting operation. The reason for this is that it is difficult to form a deep and narrow recess-shaped groove by other working operations than the cutting operation.
However, to add the above-mentioned cutting operation to the manufacturing process of the dynamic pressure bearing device not only means to add a step which is completely separate from the other steps of the present manufacturing process, but also raises a problem that the cutting operation step itself cannot provide a good operation efficiency, thereby lowering the productivity of the dynamic pressure bearing device greatly, which leads to the high cost of the dynamic pressure bearing device. More specifically, in the above-mentioned step dynamic pressure bearing device and taper dynamic pressure bearing device, if a molding operation or the like is employed, then the main portions and dynamic pressure generating portions 2a of the dynamic pressure bearing devices can be manufactured at a low cost and with high efficiency; but, if the above-mentioned cutting operation step of cutting the negative pressure cancel portions 2b respectively consisting of the above-mentioned recess-shaped separate grooves are added, then the production costs of the these bearing devices increase greatly.